There is an increasing need for protection against the penetration of contaminants into a sheltered space, given the threat of military attacks and acts of terror using nuclear, chemical or biological (NBC) means of warfare. Such protection is provided by the use of shelters and protected spaces located inside buildings. Shelters are provided with professional NBC air filtration systems including blowers, which create an overpressure inside the protected space and provide cleaned and filtered air. The overpressure is required in order to ensure an unequivocal direction of airflow from inside the protected space to the outside atmosphere, through ever-present leaks in the walls of the shelter, the door and the window seals. Use of this method assures that no contaminated air will penetrate into the protected space. Due to the potential of terror attacks shelters alone can no longer provide the degree of protection to be used as protected space, but complete facilities or parts of buildings have to be pressurized using airtight windows and other required elements. Pressurized buildings are also required for special applications such as isolation purposes.
In view of the unfortunate improvements in warfare and warfare agents, the need for more efficient airlocks which can reduce the minimum time of stay within the airlock, becomes an important requirement. Especially in the case of “silent terror attacks”, where no alert is given and the threat is not obvious to the population and even the experts, complete buildings have to be protected and used all times under conditions such as in a terror attack. For such applications airlocks of the kind commonly used in shelters are not applicable. In such airlocks both doors could not be open at the same time—either the exit or the entrance door has to be closed during times when the other door is opened. Due to the fact that thousands of persons pass through such airlocks each day in stores, in office buildings, etc., the system of airlocks and the like have to be modified, in order to provide a new level of public safety.